Thames Water bills could rise considerably over the next decade, the company has warned. Photograph: Alex Segre/Alamy

Bills for 15 million water customers in London and the south-east are to increase by up to £80 a year by the next decade after the government approved the capital's £4.2bn "super sewer" project.

Communities secretary Eric Pickles and environment secretary Elizabeth Truss gave planning permission for the 15-mile Thames Tideway tunnel, which will run the length of the river through London.

"This is a challenging infrastructure project, but it is clear that the Thames tunnel will help modernise London's ageing Victorian sewerage system and make the River Thames cleaner and safer," said Pickles.

Work on the tunnel, almost as wide as three double-decker buses, will begin in 2016 and is expected to be complete in 2023.

Thames Water said the final cost would add £70-£80 a year to bills under a "worse-case scenario" for its 15 million customers, as far apart as Oxford and Kent. The company argues the tunnel is necessary to prevent millions of tonnes of sewage spilling into the Thames during heavy downpours.

But a group of independent experts, including six university professors, said the government had failed to consider cheaper, greener alternatives. "There is no need for a Thames Tideway Tunnel; it is unnecessary and will cause extensive damage to the environment by losing the opportunity of a better solution," said the open letter.

"We have heard and scrutinised all the evidence and arguments put forward by [Thames Water] and found it a project par excellence of a government licensed monopoly company putting private profit for its shareholders above the primary, basic, public and essential need of its customers."

Thames Water is owned by a consortium led by the European arm of the Australian bank Macquarie Group.

One signatory, Colin Green, professor of water economics at Middlesex University, said the project was out-dated and similar plans had been abandoned by other cities years ago. "We are marching backwards into the future."

He accused Thames of misrepresenting the sewage problem, which he said was confined to small areas of London, such as Beckton and Hammersmith, and could be tackled by more efficient water management for half the cost.

"Thames Water has been brilliant about creating a misrepresentation of the problem," he said. "I go beserk when I see [the company claiming] that 8 bn toilets are flushed into the Thames."

Although sewage was being discharged into the Thames, the amount was much less than Thames claimed, he said. A mix of policies, from better urban drainage, to planting shrubs on rooftops in order to absorb rain and using rainwater to flush toilets, would be a cheaper, greener alternative to the super sewer, he argues.

Dido Berkeley, director of Thamesbank, a group that campaigns for the protection of the Thames, accused ministers of ignoring independence advice. "The government is rubber-stamping the great drain robbery."

A spokesperson for Thames Water said: "Years of study have shown that all the suggested alternatives to the Thames Tideway Tunnel would be ineffective, more expensive, more disruptive, take longer, or all four."